The invention relates to power demand control in general, and more particularly to a power demand control system and method for shedding and/or restoring loads in a plant so as to maximize the consumption of energy within a limit imposed in the power demand consumed by the plant under the billing rates of the utility company when they are computed in accordance with a "floating" or "sliding" window, namely, a continuous power integration through a sliding demand period, rather than by the conventional accumulation of energy through successive constant demand periods. The invention more specifically relates to plant installation where there are only a few controllable loads and where such few loads are relatively large loads that cannot be shed and restored very frequently.
Plant management requires constantly that decisions be made to curtail or increase loads selectively so as to maximize profits, to optimize performance, and/or trim power peaks according to the time of the day and the overall plant activity under a given demand limit established with the electric utilities.
Automatic load control in a plant depends upon how the utility companies are charging the customer per kilowatt-hour consumed and per accumulated kilowatt-hour within the demand period (the kilowatt-hour/hour average rate measured during the time period). There are five basic control modes depending on the rate applied to the customer:
(1) An instantaneous demand controller--This controller is based on straight line accumulation, e.g., constant usage. The rate of accumulation is imposed as a setpoint for load switching in the overall plant.
(2) An ideal rate controller--This mode of control accepts an offset point or base consumption by non-discretionary loads, thereby reducing cycling operation.
(3) A converging rate controller--Here the accumulated usage is set to converge toward an assigned demand limit, thereby to relieve control in the beginning of the period and tighten control toward the end of the period.
(4) The predictive demand controller which is typified by U.S. Pat. No. 3,872,286 of Richard Putman. Here, the energy usage and usage rate are observed periodically through a fixed demand period. By projection to the end of the demand period of predicted usage is calculated. This is compared in the usage corresponding to the demand limit, establishing an error which determines whether to shed or restore loads.
(5) A continuous integral controller--The determination of demand is effected by averaging the power utilized within a time interval ("window"), the start and ending points of which are made to continuously move ("slide") in time, rather than through fixed consecutive adjacent time intervals. Thus, each consecutive demand period is slightly shifted from the preceding one and from the following one. If the demand window is say fifteen minutes, the successive demand periods may be considered to start every four seconds, thus allowing 250 demand periods to start (and another 250 to end) within a 15 minute time span. In this regard, see "An Update on Rate Reform and Power Demand Control Load Data Working Group" by K. Chen and E. Palko in 1979 IEEE Vol. IA-15 No. 2 March/April 1979 pp. 214-220.
The present invention relates to a continuous integral controller system. In order to reduce the frequency of switching loads ON and OFF, prior art may use satellite cycle timers on the loads or groups of loads. Once activated, the satellite timer will shed loads through a complete cycle and overlap successive demand intervals, thereby reducing short cycle operation.
Certain loads, however, especially large production loads, such as electric arc furnaces, cannot be manipulated too often.
The prior art method of sliding window demand control, while suitable for many small non-production loads, could then be quite detrimental to plant operation, and this method computing the demand by the utility company will unduly penalize the user.